I just had the Topped Toys gape keeper 140 in which is supposedly 4.5" wide - but, being soft plat silicone, it squishes. I know with harder toys like the Domestic Partner B52 plug which is 4" almost exactly at max, that my butt muscles (cheeks) themselves are being squished between the plug and my pelvis and i feel them get kneaded hard as the plug goes in - which is a bit uncomfy and unrelated to the stretch.
I can stretch additional in front for eg, I got the Tantus tex (3.1 head / 2.1 neck) and the Yiff Monkey Cyclops medium plug (3.2 head / 2.2 neck) in at the same time! That was pretty intense. I can also get the classic Doc Johnson 3" dildo (2.8" shaft) in with a 2.75" realskin pretty easily, stretching more front to back than at the sides. So I know with the gapekeeper Im squishing it into an oval when it goes in.
So my pelvis is literally around 4.0 or so, with discomfort of ‘rolling’ or ‘kneading’ the bottom of my glutes, but the most I've had in is a gapekeeper 140 at 4.5". It's quite hard to get in, since it has to be squished in, need the right hardness of surface and angle, gotta rock around squeeze and release, bounce. (Need a partner putting additional weight on my shoulders! Should ask my wife…)
Cant keep it in long. Too many other organs competing for space 😜 Feels as good coming out as going in, the release is amazing.
As for fissures, etc, no real problems unless you go to hard (sometimes with double penetrations I can get a bit of spotting if im not gentle, especially with ‘pointier’ and harder plugs/dildos.
As for @Tatum - could be your pelvis, and I wouldnt use harder toys. Softer, but then get on a soft surface (bed? I have to pull below the mattress with hands up into me while bouncing to get it in) - and stretch a lot first. Using the MrsAndMrsDildo Large comfort butt plug (3.75" max) in medium hardness getting it to slide in and out easily isnt really enough stretch for the gapekeeper to go in easily, and even though the Bad Dragon Egg XXL is 4.2", it's very soft too and requires same effort. Only the PVC Titan 4" gets me ready, but getting ready for it is the problem too (and it's quite hard material, it can be quite hard on the insides, I dont like using it - wish I had a soft silicone version of it)